Systems for manufacturing commercial products of sintered magnet include a single part system wherein a part of substantially the same shape as the product is produced at the stage of press forming, and a multiple part system wherein once a large block is formed, it is divided into a multiplicity of parts by machining. When it is desired to manufacture parts of small size or parts having a reduced thickness in magnetization direction, the sequence of press forming and sintering is difficult to form sintered parts of normal shape. Thus the multiple part system is the mainstream of sintered magnet manufacture.
As the tool for cutting rare earth sintered magnet blocks, a grinding wheel outer-diameter (OD) blade having diamond abrasive grains bonded to the outer periphery of a thin disk as a core is mainly used from the aspect of productivity. In the case of OD blades, multiple cutting is possible. A multiple blade assembly comprising a plurality of cutoff abrasive blades coaxially mounted on a rotating shaft alternately with spacers, for example, is capable of multiple cutoff machining, that is, to machine a block into a multiplicity of parts at a time.
The current desire for more efficient manufacture of rare earth sintered magnet entails a propensity to enlarge the size of magnet blocks to be cutoff machined, indicating an increased depth of cut. When a magnet block has an increased height, the effective diameter of the cutoff abrasive blade, that is, the distance from the rotating shaft or spacer to the outer periphery of the blade (corresponding to the maximum height of the cutoff abrasive blade available for cutting) must be increased. Such larger diameter cutoff abrasive blades are more liable to deformation, especially to deflect on axial direction. As a result, a rare earth magnet block is cut into pieces of degraded shape and dimensional accuracy. The prior art uses thicker cutoff abrasive blades to avoid the deformation. Thicker cutoff abrasive blades, however, are inconvenient in that more material is removed by cutting. Then the number of magnet pieces cut out of a magnet block of the same size is reduced as compared with thin cutoff abrasive blades. Under the economy where the price of rare earth metals increases, a reduction in the number of magnet pieces is reflected by the manufacture cost of rare earth magnet products.
While there is a desire for the method for cutoff machining a magnet block having an increased depth of cut without increasing the effective diameter of cutoff abrasive blades, a method involving sawing an upper half of a magnet block, turning the block upside down, and sawing a lower half (upper half after the upside-down turning) of the magnet block is known. This method is successful in reducing the effective diameter of cutoff abrasive blades to about one half, as compared with the method of sawing a magnet block in one direction, and thus overcomes the above-discussed problems of dimensional accuracy and the width to be sawn associated with thick blades, but needs strict alignment of the cutting position before and after the upside-down turning. The step of alignment of the cutting position takes a time. If the cutting position is misaligned even slightly, a step is formed between upper and lower cutoff surfaces. If so, the step must be eliminated or smoothened by surface grinding after the cutoff machining. When cutoff machining is continuously performed as is often the case in commercial manufacture, it is impossible in a substantial sense to cutoff machine all magnet blocks without leaving a step between upper and lower cutoff surfaces. Thus a magnet block is typically sawn into slightly thicker pieces, with an allowance for surface grinding being taken into account. The number of magnet pieces cut out of a magnet block of the same size is reduced in this case too.